Four Moroccans arrested by Italian police in Rome allegedly had maps showing the city water supply, the US embassy, and a substance apparently containing small quantities of cyanide, officials said.

Chief prosecutor Salvatore Vecchione said the substance, which was undergoing analysis, appeared to be potassium ferrocyanide.

Mr Vecchione was outraged that news of the raid, which took place on Tuesday on the outskirts of the capital, had leaked out and said it may have badly damaged an ongoing investigation.

The Italian media was full of speculation about the case and the suspects.

The AGI news agency said the men were part of the Salafist Group for Call and Combat, a dissident faction of the Armed Islamic Group, Algeria's most radical insurgent group.

Other reports linked at least one of the suspects to a terrorist cell in Milan suspected of having links to Osama bin Laden's al Qaida network.

The cell was dismantled last year with the arrest of seven Tunisians, who are now on trial in Milan. In wiretapped phone conversations, the Tunisians appeared to be speaking in code about cyanide.

Officials refused to comment on most of the reports. Mr Vecchione, who said his office was investigating the leaks, refused to go beyond confirming the arrests and the discovery of maps, false documents and the chemical substance.

News reports said the maps showed the city's water supply, along with the embassy.

"They had the US embassy circled and I don't think they were planning to pick up tourist brochures," said a source familiar with the investigation.

"Because of their ongoing commitment to countering the terrorist threat, Italian authorities have repeatedly thwarted planned terrorist attacks against American and other targets inside Italy," he said.

A terrorist threat closed the embassy for three days in January 2001, the first closure in about a decade. Mr Boucher said no closure is expected because of Tuesday's arrests. "At this point we don't see an immediate threat to the embassy or embassy employees," he said.

News reports said investigators believed the suspects planned to contaminate water supplies in the capital, including the commercial area around Via Veneto where the US Embassy is located.

Tuesday's pre-dawn raid was described as the biggest probe so far into suspected Islamic terrorist activity in the city. Three other Moroccans were arrested in the capital last week as part of the same investigation, reports said.

Ferrocyanide is a common industrial chemical used in the production of wine and ink dye, among other things. Luciano Caprino, a pharmacology professor at Rome's La Sapienza University, said small amounts of cyanide can be extracted from it, but with great difficulty.